General Chess Opening Confusions.

I'm having confusion understanding the opening phase in chess. They say there are three phases in the chess game. 1. Opening, 2. Mid-Game and 3. End game but I don't understand when the opening phase ends and the "Mid-Game" phase starts and similarly when the mid-game phase ends and the end-game phases starts. i.e after how many moves does the opening phase ends ? e.g I say after exactly 10 moves, the opening phase ends and the mid-game phase is of 15 moves and the rest is the end-game phase, that makes "sense" but I don't understand how can you say just by looking on the position of the board whether it's the "opening" phase or "mid-game" phase or "end-game" phase. Is there an exact way to know which phase you're in or is this only pridiction and theory ? Also I wanted to ask if you can respond to a particular opening without the corresponding defense ? Of course you can, but I meant if you do, what defense it will be considered according to the ECO ?

for instance, white moves 1. d4 and then black plays 1. c5 now that's considered as sicilian defence according the ECO. What if white plays 1. a4 and then black plays 1. c5, will it still be considered as sicilian defense ? Yes ? No ??

And what if white moves 1. c4 and then black plays 1. c5, why it is considered as Symmetrical Variation instead of Sicilian Defense according to the ECO ??? I'm damn confused. My poor brain can't think, please help!!!

The primary objective of the opening phase is to deploy and to mobilize your forces. Once your forces have been deployed to their desired positions, the opening phase ends. There is no exact number of moves for this. It is possible that you may have completed your opening phase while your opponent hasn't. Middlegame starts when one or both players are ready to attack. Often you may be able to win in the middlegame without going to the endgame phase. When the battle is long drawn and the number of pieces are greatly reduced, the endgame starts. There is no clear definition of when the endgame begins. Different people seem to have their own definitions. My own personal definition is that the endgame begins when you can no longer deliver a mate without resorting to pushing your pawn towards promotion. About the ECO thing, don't worry about it. It's just a classification. It has changed in the past, and it may change in the future. The most important thing is to know when to use and what to use.